Double throw pannier, [ but Ive never held one in my hands] .
http://bontrager.com/model/09563
so I cant say about the over the shoulder thing.
The addition of some webbing, joined my pair of Ortlieb sport packers,
but the hooks in the back of the bags would hurt digging into your shoulder.. as they are,
But, the same pannier set is used as their Horse Pannier,
but those lack any way to attach it to the bike's rack
since the hooks are eliminated , to not dig into the back of the horse, behind that saddle .
the Marhall Dillon thing may be a odd combo.
know anyone with a Sewing Machine? might be time to make the item you seek.